The present invention relates to electrical lighting devices and systems, and more particularly to lamps that include light emitting diodes (LEDs) but that resemble linear fluorescent lamps.
An LED is an electroluminescent diode that includes a semiconducting material doped with an impurity to form a p-n junction. When a forward bias is applied to the electrodes of the diode, charge carriers flow into the junction and recombine emitting photons. The energy of the photon, and therefore the wavelength of the light, depends on the band-gap energy of the materials forming the p-n junction. Materials for the LEDs can be selected so that the emitted light has a particular color, such as blue, green, amber, or red.
White LEDs often include a blue LED coated with a YAG:Ce phosphor. High power (one watt or more) blue LEDs are about 30-45% efficient, with about 550-700 mW going into heating the device for each watt applied. In addition, the phosphor conversion of blue light into yellow light in a white LED accounts for about 20% of the incident energy, which goes into heating the phosphor. LED technical specifications indicate that blue LEDs have a blue light power depreciation of about 7% (temperature 25-125° C.) while white LEDs have a power depreciation of about 20% at the same temperature. Thus, high power white LEDs impose significant thermal and lumen maintenance constraints.
Tubular lamps that use white LEDs are known. However, the LEDs appear as point sources along the length of the tubular cover so the light is not uniform. Some lamps use a refractive cover to the diffuse the light to achieve a more uniform illumination. It is also known to series-connect plural blue LEDs on a PCB board.
Various solutions are proposed in U.S. Pat. Nos. 5,463,280; 5,688,042; 5,949,347; 6,036,336; 6,283,612; 6,583,550; 6,634,779; 7,114,830; 7,249,865; 6,762,562; 6,796,680; and 6,940,101.